I am working on a wheel speed sensing and traction control system as part of
our schools Formula SAE race car. Basically, it has more power than traction
(about 80HP/500 pounds) and we don't have much time to practice driving it,
so the more help we can get in the traction department, the better
(especially in the acceleration/drag race event).

The basic principle is to compare the speeds of the front (non driven) and
back (driven) wheels to determine how much slip/loss of traction there is.
Apparently, about 10% slip is best. If the back wheels are spinning too
fast, the throttle is cut electronically (strict throttle by wire is
prohibited for obvious reasons, but just cutting fuel/spark isn't) to limit
the slip (and hopefully maintain it with decent control).

I plan to use a CAN bus around the car, both for the wheel speed sensing,
and other data transfer (shifting information, other sensors in the future).
I have designed a wheel speed sensor (there will be 4 copies of it) that
will mount on the kingpin assembly - it has a hall effect sensor, PIC10F or
12F (timing pulses), MCP2515 CAN controller, and a CAN transceiver IC that
hangs on the bus. Hopefully the CAN transceiver will make it pretty easy -
all it needs is SPI data from the PIC.

The other end will be a bit more difficult. I still haven't worked out the
algorithms yet. I think it will likely consist of a similar CAN interface
feeding a 16F or 18F PIC (some of the 18's also have a built in CAN
interface). This PIC will have 4 outputs so it can individually cut
cylinders. I think this PIC should probably be done in C to make it easier
to try new algorithms.

I would appreciate comments on the system outlined above.

What I am wondering about the most is the feedback loop. I don't have much
experience dealing with feedback systems. What I am thinking is that it
would probably be a good idea to measure the loop gain and bandwidth of the
system once we have the hardware put together. We need to know how fast the
engine will respond to cutting differing amount of cylinders for a given
amount of time (it seems that this will change with engine speed and load
also). Or is this too much to keep track of and it would be better just to
do a trial and error algorithm?

Thanks for any help/advice you can give.

Andy Meng

LeTourneau University
Formula SAE Race Team

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